24.
Random Perihelion Directions,
Orbit Directions and Inclinations. A few, brief, volcanic eruptions from planets or moons would launch primarily prograde comets in specific directions with similar orbital planes and perihelion directions. Instead, about half the long-period comets are retrograde and have randomly oriented orbital planes and perihelions.

The most violent volcanic eruption seen anywhere in the solar system occurred not on Earth, but on Io (EYE-oh), a moon of Jupiter. The energy released was less than a thousandth of that needed to launch even a few comets from Io. Besides, Io was expelling sulfur dioxide, not water.108 Volcanic eruptions would lose too much energy in passing up through narrow conduits and vents. High pressures can only build up in a solid—not in a gaseous planet.

25.
Small Perihelions. Long-period comets have perihelions concentrated in the 1–3 AU range. Had they been launched from a giant planet (those lying 5–30 AU from the Sun), their perihelions would be farther from the Sun.

26.
High Loss Rates of Comets. Vsekhsvyatsky, this theory’s leading advocate, by assuming billions of years of comet accumulation, estimated that at least 1020 grams of comets are expelled from the solar system each year.109 Other cometary material should have been lost by evaporation and collisions. On Earth, all volcanoes combined eject only about 3 Ã 1015 grams of material into the atmosphere each year.110 Therefore, according to this theory, cometary material is being lost from the solar system thousands of times faster than Earth’s volcanoes are ejecting material only a few miles above Earth’s surface.

Matter expelled from a planet or moon might later collect gravitationally into a comet if a large amount of it traveled together. However, volcanoes eject small amounts of matter over wide angles. Ejected material must also travel far enough from the planet to have a large sphere of influence. For the giant planets, this is difficult. Jupiter’s escape velocity, for example, is 38 miles per second. Astronomers have never seen matter being permanently expelled from a giant planet.

27.
Composition,
Heavy Hydrogen. The giant planets are primarily gas—hydrogen and helium. Those planets do not have the higher concentrations of the heavier elements that are in comets. The ratio of heavy hydrogen to normal hydrogen in comets is 20 times greater than in Jupiter and Saturn. If oxygen, carbon, silicon, magnesium, nitrogen, sodium, and other relatively heavy elements in comets came from any giant planets, they must have come from deep within, where they would sink. Eruptions from deep within gaseous planets would be easily suppressed by viscous drag. If comets came from any giant planets or their barren moons, why would comets have organic compounds, such as methane, ethane, and the amino acid glycine or minerals that form only in the presence of hot liquid water? This theory does not explain any of the six discoveries of the Deep Impact mission listed on page 295.